1. Field of the Invention
This invention relates in general to pressurized water nuclear reactors and, in particular, to systems for injecting additional coolant into the reactor coolant circuit in the event of a postulated accident. The invention is applicable to reactor systems having passive safety features with automatic depressurization of the reactor coolant circuit to facilitate the injection of additional coolant water.
2. Related Art
A nuclear reactor, such as a pressurized water reactor, circulates coolant at high pressure through a coolant circuit traversing a reactor pressure vessel containing nuclear fuel for heating the coolant and a steam generator operable to extract energy from the coolant for useful work. A residual heat removal system is typically provided to remove decay heat from the pressure vessel during shutdown. In the event of a loss of coolant, means are provided for adding additional coolant. A coolant loss may involve only a small quantity, whereby additional coolant can be injected from a relative small high pressure make-up water supply, without depressurizing the reactor coolant circuit. If a major loss of coolant occurs, it is necessary to add coolant from a low pressure supply containing a large quantity of water. Since it is difficult using pumps to overcome the substantial pressure of the reactor coolant circuit, e.g., 2,250 psi or 150 bar, the reactor coolant circuit is depressurized in the event of a major loss of coolant so that coolant water can be added from an in-containment refueling water storage tank at the ambient pressure within the nuclear reactor system containment shell.
The primary circuit of an AP1000 nuclear reactor system, offered by the Westinghouse Electric Company LLC, of which the present invention is a part, uses a staged pressure reduction system for depressurizing the primary coolant circuit, which is illustrated in FIGS. 1 and 2. A series of valves 72 couple the reactor outlet 56 (also known as the “hot leg” of the primary coolant circuit) to the inside of the containment shell 54. When initially commencing the pressurization, the coolant circuit 46 and the containment structure 54 are coupled by the depressurization valve 72 through one or more small conduits 76 along a flow path with not insubstantial back pressure. As the pressure in the coolant circuit drops, additional conduits are opened by further depressurization valves 72 in stages, each stage opening a larger and/or more direct flow path between the coolant circuit 46 and the containment shell 54.
The initial depressurization stages couple a pressurizer tank 80 which is connected by conduits to the coolant circuit hot leg 56, to spargers 74 in an in-containment refueling water supply tank 50. The spargers 74 comprise conduits leading to small jet orifices submerged in the tank, thus providing back pressure and allowing water to condense from steam emitted by the spargers into the tank 50. The successive depressurization stages have progressively larger conduit inner diameters. A final stage has a large conduit 84 that couples the hot leg directly into the containment shell 54, for example, at a main coolant loop compartment 40 through which the hot leg 56 of the reactor circuit 46 passes. This arrangement reduces the pressure in the coolant circuit expeditiously, substantially to atmospheric pressure, without sudden hydraulic loading of the respective reactor conduits. When the pressure is sufficiently low, water is added to the coolant circuit by gravity flow from the in-containment refueling water supply tank 50.
Automatic depressurization in the AP1000 reactor system is a passive safeguard which ensures that the reactor core is cooled even in the case of a major loss of coolant accident such as a large breach in the reactor coolant circuit. Inasmuch as the in-containment refueling water storage tank drains by gravity, no pumps are required. Draining the water into the bottom of the containment building where the reactor vessel is located, develops a fluid pressure head of water in the containment sufficient to force water into the depressurized coolant circuit without relying on active elements such as pumps. Once the coolant circuit is at atmospheric pressure and the containment is flooded, water continues to be forced into the reactor vessel, where the boiling of the water cools the nuclear fuel. Water in the form of steam escaping from the reactor coolant circuit is condensed on the inside walls of the containment shell, and drained back to be injected again into the reactor coolant circuit.
The foregoing arrangement has been shown to be effective in the scenario of a severe loss of coolant accident. However, there is a potential that if the automatic depressurization system is activated in less dire circumstances, the containment may be flooded needlessly. Depressurization followed by flooding of the reactor containment requires shut down of the reactor and a significant cleanup effort. This concern has been partially addressed in U.S. Pat. No. 5,268,943, assigned to the Assignee of this invention.
It has been postulated that a spurious actuation of the AP1000 automatic depressurization system under normal conditions could lead to an accident that is more severe than has been analyzed for the plant. Accordingly, a further improvement in the automatic depressurization is desired to guard against such an occurrence.
Therefore, it is an object of this invention to provide a device that blocks actuation of the automatic depressurization system valves under normal plant conditions.
It is further object of this invention to provide such a device that will maintain a blocking signal on the inputs of the depressurization system when the core makeup tanks are full, to reduce the initiating event frequency of spurious automatic depressurization system actuation. In true accident scenarios, the core makeup tanks are drained in the early stages of the mitigation. Therefore, low level in either of these tanks will provide an indication that the blocking signal needs to be removed to allow the safety system to actuate the automatic depressurization system valves as designed.
Further, it is an object of this invention to provide such a system that is substantially fail safe to assure that it does not impede the actuation of the automatic depressurization system when it is needed.